This invention relates to electrophotographic copiers, and more particularly to an apparatus for lubricating or removing toner particles from a heated fuser roller. In the art of electrophotography, the usual method of carrying out a copying process is to electrostatically charge a surface and then expose it to a light pattern of an image to be reproduced, thereby discharging the charge in the areas where the light strikes the surface. The latent electrostatic image is then developed by contacting it with a finely divided electrostatically attractable powder material known in the art as a toner. The toner is held in the image areas not exposed to light which have retained the electrostatic charge. The powder image is then transferred to the surface of the final support material, such as paper, in paper copying. The support material with the powder thereon must then be fed to a fusing station, to permanently fuse the powder material onto the support.
One means of fusing or fixing the powder material onto the support is by means of heat and pressure, provided by passing the final support through a pair of rollers maintained in pressure contact. Heat is applied to either or both rollers by conventional means.
The electrostatically attractable powder, may be made of a wide variety of materials, but toners comprising a pigment such as carbon black dispersed in a thermoplastic material are particularly suited for this process. Typical toner powders are finely divided to permit electrostatic attraction and produce sharp images, and may for example consist of approximately 5 to 10% carbon black dispersed in a polystyrene thermoplastic binder. Complete transfer of such toners from the electrostatically charged surface to the paper, and complete retention by the paper through the fusing or fixing process have been difficult to maintain.
For example, a common method of transferring the dry image powder from the electrostatic surface to the support is by means of a developing drum, which in one rotation is charged, exposed to develop a latent image, developed with toner, and then brought into contact with the final support for the transfer of the imaged toner onto the support. In such a process, it has been necessary to provide means for cleaning the drum after toner transfer due to incomplete transfer of the toner onto the support. A wide variety of devices have been employed to clean residual toner from the electrostatic surface of the drum after transfer. These include brushes, webs, or squeegees which may be made of cross-linked polymeric materials, particularly elastomers, and may be in the form of a roll. For example, Kolb et al., U.S. Pat. No. 3,299,787, discloses a squeegee roll which may be formed from a natural or synthetic rubber to remove residual toner. Ruhland, U.S. Pat. No. 3,781,107, employs polyurethane foam for wiping of the developing drum. Since the toner is still in solid form and has not been subjected to sufficient heat to cause softening of the thermoplastic binder, a wide variety of devices and materials have been used with success to remove the dry powdered toner from the developing drum.
In contrast, through the fusion or fixing process, the toner is subjected to sufficient heat and pressure to cause softening of the thermoplastic binder and fusion onto the paper. The prior art has been plagued with problems in heat-pressure fixing processes using direct contact rollers in that the roller tends to pick up some of the fused toner from the paper and deposit it on another portion of the sheet, causing what is commonly called "offset". Due to this offset problem, many electrophotographic copying apparatuses have discarded the direct contact heated fusion roller approach and designed less efficient non-contact fixing processes such as radiant heating of the paper.
Solutions to this offset problem have been approached in two general ways. The first is the prevention of toner pickup by the fusion roller, and the second is the removal of the toner after pickup and before offset can occur. The first approach, i.e. prevention of toner pickup, has generally been accomplished by fabrication of the fuser roller with a material having high surface release property such as silicone rubber. For example, the use of a silicone elastomer blanket about the fusion roller, as in U.S. Pat. No. 3,669,707. Although such high surface release rollers have minimized the amount of toner pickup from the paper, they have not completely eliminated the problem. Thus, it has been found that additional lubrication of such rollers is necessary to eliminate the toner pickup for extended periods. For example, in U.S. Pat. No. 3,268,351 to Van Dorn, silicone oil is applied to a high release teflon fusion roller. A separate oil reservoir feeds a saturated felt pad, which in turn feeds an applicator roller which continuously lubricates the fusion roller. Such a device, although effective in preventing toner pickup while lubricated, suffers from the disadvantage that it requires constant maintenance to insure that there is an adequate supply of lubricating oil in the reservoir. Intricate oil reservoirs and wick assemblies have been devised as for example U.S. Pat. No. 3,745,972, which teaches a teflon-Nomex wick assembly, to increase the efficiency of the system. Nevertheless, once the lubricant is depleted, offset rapidly occurs.
The second approach recognizes that a certain amount of toner will adhere to the fusion roller, but attempts to alleviate the problem by providing a means of cleaning and removing the toner from the fusion roller surface. Unlike the cleaning of the developer drum where the powder is in dry, fine particulate form, loosely held to the developing drum by electrostatic force, in the cleaning of the fusion roller, the thermoplastic binder of the toner has been caused to melt and thereby form a more permanent bond with the fusion roller. Removal of the hot, coalesced toner material from the hot fusion roller is a much more difficult problem than that of cleaning a developer roller. For example, in U.S. Pat. No. 3,649,992 to Thettu, metal cylinders having a high thermal conductivity are employed to rapidly transmit the heat energy from the toner to the cylinders, which in turn causes the toner to solidify and transfer from the fusion roller to the metal cylinders. Such a cleaning apparatus, used in conjunction with a high release surface fusion roller has been found effective to prevent toner offset for extended periods. However, depending upon the operating conditions of the reproduction machine, offset still begins to appear after several thousand copies. Replacement of the metal rollers can be a costly and time consuming process. Although it is not fully understood under what set of condition offset begins to occur in such a system, it is believed that the high release surface fusion roller, after extended use, loses its release properties, and the metal roller eventually becomes sufficiently abraded and contaminated as not to be capable of removing all of the toner.
It is thus an object of this invention to provide a heat-pressure roller fusing system for fixing electrophotographic copies, which will operate for longer periods of time without offset than the systems of the prior art, and without the use of lubricating reservoirs.
It is another object of this invention to provide a fusing system wherein when offset begins to occur, it may be readily and inexpensively remedied in the field.